1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a reflective LCD device having a cholesteric liquid crystal (CLC) color filter layer.
2. Discussion of the Related Art
Flat panel display devices, which have properties of being thin and low weight and having low power consumption, are in high demand in the display field as the information age rapidly evolves.
The flat panel display device may be classified into two types depending on whether it emits or receives light. One type is a light-emitting type display device that emits light to display images and the other type is a light-receiving type display device that uses an external light source to display images. Plasma display panels, field emission display devices and electro luminescence display devices are examples of the light-emitting type display devices and liquid crystal displays are examples of the light-receiving type display device. The liquid crystal display device is widely used for notebook computers and desktop monitors because of its superiority in resolution, color image display and quality of displayed images.
Generally, the liquid crystal display (LCD) device has upper and lower substrates, which are spaced apart and face each other. Each of the substrates includes an electrode and the electrodes of each substrate face each other. Liquid crystal is interposed between the upper substrate and the lower substrate. Voltage is applied to the liquid crystal through the electrodes of each substrate, thereby changing an alignment of the liquid crystal molecules in accordance with the applied voltage to display images. Because the liquid crystal display device cannot emit light alone as described before, it needs an additional light source to display images. Accordingly, the liquid crystal display device has a backlight device as a light source behind a liquid crystal panel. An amount of incident light from the backlight is controlled in accordance with the alignment of the liquid crystal molecules to display images.
The electrodes of each substrate are formed of transparent conductive material and the substrates must be transparent. This type of liquid crystal display device is called a transmissive LCD device. Because the transmissive LCD device uses an artificial light source such as the backlight device, it is possible to display a bright image under dark conditions. However, the transmissive LCD device has high power consumption.
A reflective LCD device has been suggested to overcome the high power consumption problem of the transmissive LCD device. In the reflective LCD device, an opaque and reflective metallic material is used as a pixel electrode instead of the transparent conductive material. Thus, the pixel electrode made of reflective material reflects the light toward its incident direction to display images depending on the alignment of the liquid crystal molecules, and the reflective LCD device has a low power consumption compared with the transmissive LCD device. Additionally, an electrode of the upper substrate is formed of transparent conductive material to transmit the incident light.
The conventional reflective LCD device will be described hereinafter in more detail with reference to FIG. 1. FIG. 1 is a cross-sectional view of a conventional reflective liquid crystal display device. In FIG. 1, a plurality of switching elements (not shown) are disposed in the form of an array matrix on a first substrate 1, and a plurality of reflective electrodes 3 each of which is connected to each of the switching element are formed on the first substrate 1. The reflective electrode 3, which is formed of a conductive material such as metal, serves as a pixel electrode to reflect the incident light. A color filter 4 that includes sub-color-filters red (R), green (G), and blue (B) in a repeated order is formed on the rear surface of a second substrate 2 and corresponds to the reflective electrode 3. A common electrode 5 is formed of a transparent conductive material on the color filter 4. A liquid crystal layer 6 is interposed between the reflective electrode 3 and the common electrode 5 such that an alignment of liquid crystal molecules changes when a voltage is applied to the reflective electrode 3 and the common electrode 5. Although not shown in FIG. 1, alignment films (not shown) may be formed on the reflective electrode 3 and on the common electrode 5, respectively, to align the liquid crystal molecules into a predetermined direction.
A retardation layer 7 is formed on the front surface of the second substrate 2. The retardation layer 7 herein has a retardance of λ/4 and is also called a quarter wave plate (QWP). The quarter wave plate 7 converts a linearly polarized light into a circularly polarized light and vice versa. A polarizer 8, which changes ambient light into linearly polarized light by way of transmitting only light components that are parallel to the optical axis of the polarizer, is formed on the quarter wave plate 7.
If the ambient light is irradiated on the reflective liquid crystal display device when there is no voltage applied, the incident light is changed into linearly polarized light as it passes through the polarizer 8, and the linearly polarized light is changed into circularly polarized light as it passes through the quarter wave plate 7. The circularly polarized light then passes through the second substrate 2, the color filter 4 and the common electrode 5 in sequence and without the polarization. The circularly polarized light then passes through the liquid crystal layer 6. When the light passes through the liquid crystal layer 6, the circularly polarized light is converted into linearly polarized light if the liquid crystal layer 6 has a phase difference of λ/4. The linearly polarized light is reflected on the reflective electrode 3 and then is changed into circularly polarized light as it passes again through the liquid crystal layer 6. The circularly polarized light is changed into linearly polarized light as it passes again through the quarter wave plate 7 and then the linearly polarized light passes through the polarizer 8. When the light passes through the polarizer 8, if a polarizing direction of the linearly polarized light is parallel to the optical axis of the polarizer 8, all of the linearly polarized light transmits through the polarizer 8, and if the polarizing direction of the linearly polarized light is perpendicular to the optical axis of the polarizer 8, the linearly polarized light cannot transmit through the polarizer 8.
Meanwhile, cholesteric liquid crystal (CLC) display devices, which use the cholesteric liquid crystal (CLC) as a color filter to display color images, has been widely researched and developed in the field of LCD devices. A reflective CLC display device, which has a CLC color filter, is known to have a superior color reproduction and contrast ratio compared with a typical reflective LCD device that has an absorption type color filter. The CLC color filter uses a selective reflection property of the cholesteric liquid crystal. Namely, the cholesteric liquid crystal (CLC) reflects light having a certain wavelength in accordance with its helical pitch, i.e., selective reflection. That is, when all helical axes of the cholesteric liquid crystal (CLC) are aligned perpendicular to the substrate, the cholesteric liquid crystal (CLC) functions as a reflective mirror on which the incident light is reflected in the way of making the equal incidence and reflection angles with respect to a normal line to the specular surface.
However, the cholesteric liquid crystal (CLC) does not reflects all incident light but selectively reflects the incident light of a particular wavelength according to its helical pitch. Accordingly, if the helical pitch of the CLC is fixed to correspond to the red, green or blue wavelength, the CLC produces red, blue or green color. The cholesteric liquid crystal (CLC) color filter also decides a polarization state of the reflected light. If the liquid crystal molecules of the CLC are twisted counterclockwise (i.e., left-handed helical structure), the CLC reflects a left-handed circularly polarized component derived from the incident light. These characteristics distinguish the CLC from a dichroic mirror that is a mirror simply reflecting a ray of a certain wavelength and transmitting the rest of the other wavelengths (e.g., featuring infrared light reflection and visible ray transmission).
FIG. 2 is a cross-sectional view of a reflective cholesteric liquid crystal (CLC) display device that has a CLC color filter according to the related art. Because the cholesteric liquid crystal (CLC) color filter serves not only as a reflector but also as a color filter, an additional reflector is not necessary.
In FIG. 2, an absorption layer 12 is formed on the lower substrate 11 and a first alignment layer 13 is formed on the absorption layer 12. A cholesteric liquid crystal (CLC) color filter layer 14 is formed on the first alignment layer 13.
The cholesteric liquid crystal (CLC) color filter layer 14 displays red, green or blue color by reflecting light that has a wavelength corresponding to the red, green or blue color. A first transparent electrode 15 is formed on the cholesteric liquid crystal (CLC) color filter layer 14 and a second alignment layer 16 is formed on the first transparent electrode 15. A second transparent electrode 22 is formed on the rear surface of an upper substrate 21 and a third alignment layer 23 is subsequently formed on the second transparent electrode 22. A retardation layer 41 that has a retardance of λ/4 is formed on the upper substrate 21 and a polarizer 42 is formed on the retardation layer 41. A liquid crystal layer 30 is interposed between the second alignment layer 16 and the third alignment layer 23. The alignment of liquid crystal molecules changes in accordance with an electric field generated between the first transparent electrode 15 and the second transparent electrode 22.
An operating mechanism of the reflective cholesteric liquid crystal (CLC) display device, which uses a cholesteric liquid crystal (CLC) color filter, is as follows. A phase difference occurs in the liquid crystal when the voltage is applied to the transparent electrodes to generate the electric field across the liquid crystal layer.
In the normally black mode, the reflective CLC display device shows a black color when no electric field is applied to the liquid crystal layer 30. Incident light is linearly polarized as it passes through the polarizer 42 and subsequently circularly polarized as it passes through the retardation layer 41. The circularly polarized light passes through the liquid crystal layer 30 without a phase retardation and then transmits through the cholesteric liquid crystal (CLC) color filter layer 14 without a reflection, and finally absorbed by the absorption layer 12. Accordingly, there is no reflected light. Whereas when the electric field is applied to the liquid crystal layer 30, incident light is linearly polarized as it passes through the polarizer 42 and subsequently circularly polarized as it passes through the retardation layer 31. When the circularly polarized light passes through the liquid crystal layer 30, the polarization property of the circularly polarized light is changed because of phase retardation of the liquid crystal layer 30. Only the light component of a particular wavelength derived from the light transmitted through the liquid crystal layer 50 is reflected by the cholesteric liquid crystal (CLC) color filter layer 14, and the rest of the light transmits through the cholesteric liquid crystal (CLC) color filter 14 and then is absorbed by the absorption layer 12. The polarization property of the reflected light is changed as it passes again through the liquid crystal layer 30 and the reflected light is linearly polarized as it passes through the retardation layer 41. The linearly polarized light finally passes through the polarizer 42.
In the normally white mode, the reflective CLC display device shows a white color when no electric field is applied to the liquid crystal. Incident light is linearly polarized as it passes through the polarizer 42 and subsequently circularly polarized as it passes through the retardation layer 41. The circularly polarized light passes through the liquid crystal layer 30 without phase retardation. Only the light component of a particular wavelength derived from the light transmitted through the liquid crystal layer 30 is reflected by the cholesteric liquid crystal (CLC) color filter layer 14, and the rest of the light transmits through the cholesteric liquid crystal (CLC) color filter 14 and then is absorbed in the absorption layer 12. The reflected light passes again through the liquid crystal layer 30 without phase retardation and is linearly polarized as it passes through the retardation layer 41. The linearly polarized light finally passes through the polarizer 42.
When the voltage is applied to the liquid crystal, incident light is linearly polarized as it passes through the polarizer 42 and subsequently circularly polarized as it passes through the retardation layer 41. When it passes through the liquid crystal layer 30, the polarization property of the circularly polarized light is changed because of the phase retardation of the liquid crystal layer 30. All of the light transmitted through the liquid crystal layer 50 passes through the cholesteric liquid crystal (CLC) color filter layer 14 without a reflection and then is absorbed in the absorption layer 12. Accordingly, there is no reflected light.
Additionally, because the reflective liquid crystal display device uses the external light source, an incidence angle of the light varies according to a position of the light source. As described before, since the cholesteric liquid crystal (CLC) color filter creates a specular reflection, the reflection angle of the light depends on the incidence angle of the light. Whereas a luminance in a certain viewing angle is very high, the luminance in the rest of viewing angle is lowered.
In addition, since a length of the helical pitch of the cholesteric liquid crystal (CLC) helix, which the incident light experiences, is variable in accordance with the incidence angle of the incident light, the wavelength of the reflected light is changed. Accordingly, a color of the reflected light varies depending on the incidence angle of the incident light and a color change of the reflected light becomes greater as the incidence angle becomes larger.
These problems can be overcome by way of scattering the reflected light using a diffusion film over the liquid crystal panel, such that the uniform luminance in a main viewing angle range may be obtained. However, though an introduction of the diffusion film may overcome the luminance problem, there still exists a color change problem according to the incidence angle.